Customers who tap T-Mobile’s network for 5 GB of data in a month will see mobile broadband speeds reduced after that. The plan also includes voice minutes, but only 100 per month; each minute after the allotment will cost $0.10. After a few years of nearly unlimited voice minutes and offers such as free calls from one mobile to another, the 100 minutes of voice sounds low.

While heavy voice users won’t be interested in the new $30 monthly plan, it could provide a subscriber boost to T-Mobile, the No. 4 operator in the U.S. Given the pace of technology changes, two-year carrier contracts are out of sync with smartphone advances — now on a nine- to 15-month cycle or less — so a contract-free plan may appeal. Additionally, those who rely on their handsets far more for data services can do so for far less than a traditional voice and data plan that can run between $50 and $80 per month. The monthly savings could make it easier to save for a phone upgrade.

If the take-up rate for the new T-Mobile plan is strong, I wonder if any other U.S. carriers will quietly follow suit with less expensive plans that focus more on data instead of voice. It has to happen at some point: Consumers are finding more services and options to replace traditional voice calls with richer or alternative forms of communication, such as text messages, video calls and VoIP solutions. All of these use data, not voice minutes, of course.

My own smartphone usage is heavily skewed towards data, and there are now many times where I carry a 3G tablet but no phone at all. Voice calls can be forwarded to my Skype In number or callers can leave a voicemail on my Google Voice line, for example. I can send or receive text messages at will.

And although we have no landline in our house, I still rarely use my smartphone for voice calls while home. Calling phones from my computer using Google Talk’s voice calling feature in Gmail is a solution I use daily. Maybe it’s time for me to consider T-Mobile’s new data-centric plan. More importantly: Will a plan like this work for you?